This invention relates generally to the field of gas flow measurement. More particularly, it relates to a flow sensing system that displays high accuracy and stability, regardless of variations in gas composition and temperature.
Many types of gas flow sensor devices and systems have been used to measure the gas flow rate in pressurized gas systems, such as those used in life support pulmonary ventilators. One commonly used flow sensing system, known as a hot wire anemometer, employs a temperature-responsive, resistive sensor element (e.g., a heated wire or a thermistor) in the gas flow path, wherein the sensor element is electrically connected to a bridge circuit, so that a change in the gas flow rate produces a corresponding change in the temperature, and thus the resistance, of the sensor element. The resistance change, in turn, produces a corresponding change in the current flow through the sensor element that can be detected in the output signal of the bridge circuit. This output signal is then calibrated to correspond to a measure of the flow rate of the gas. See, for example, U.S. Pat. No. 2,726,546--King, Jr.
In flow sensors of the hot wire type, the current flow through the sensor element is a function not only of the gas flow rate, but also of such factors as the ambient temperature of the gas and the transport properties of the gas (e.g., thermal conductivity, density, and viscosity). Thus, changes in the ambient temperature of the gas, and in the composition of the gas (if it is a mixture of different constituent gases), can introduce errors in the flow rate measurement. Consequently, the prior art has sought to provide compensation for some or all of these error-inducing factors.
For example, one method that has been employed to minimize errors due to changes in ambient gas temperature has been to provide a constant temperature differential between a sensor element that is exposed to the flow of gas, and a similar sensor element that is disposed so as to be unaffected by flow rate changes, but which can sense the temperature of the gas. A similar method uses a first, heated sensing element and a second, unheated sensing element, both exposed to the flow stream. The changes in current needed to maintain the first sensing element at a constant temperature in response to the cooling effects of gas flow are read as a measure of the flow rate, while the changes in current through the second sensing element are read as a measure of ambient temperature, and are used to generate a compensation signal in the control circuitry. Examples of these approaches to temperature compensation, in the context of a bridge circuit arrangement, are found in the following U.S. Pat. Nos.: 3,085,431--Yerman et al.; 3,363,462 Sabin; 3,645,133--Simeth et al.; 3,648,518--Hans et al.; 3,747,577--Mauch et al.; 4,043,196--Trageser; 4,080,821--Johnston; 4,343,183--Plapp; 4,344,322--Plapp; 4,373,387--Nishimura et al.; 4,527,427--Grunwald; 4,537,068--Wrobel et al.; 4,562,731--Nishimura et al.; 4,596,140--Dorman et al.; 4,599,895--Wiseman; 4,686,856--Vavra et al.; 4,807,151--Citron; 4,845,984--Hohenstatt; 4,854,167--Czarnocki et al.; 4,905,514--Wiseman; 4,938,061--Carp; 4,982,605--Oram et al.; and 5,181,420--Suzuki et al.
Typically, in a bridge-type circuit, the sensor element comprises one of the resistances in the bridge circuit. Thus, if the flow sensor is connected to the other components of the bridge circuit by wires and connectors, the wires and connectors themselves become part of the bridge circuit, thereby introducing a further possible source of error, since these components tend not to be consistent in their electrical characteristics from unit to unit.
Thus, temperature-compensated flow sensor systems that employ circuits other than bridge-type circuits have been developed. Examples are found in the following U.S. Pat. Nos.: 4,244,217--Ledbetter; 4,297,881--Sasayama et al.; 5,072,614--Hisanaga; and Re. 33,076--Sigiura.
While these prior art arrangements yield improved accuracy, they may still be sensitive to changes in the transport properties of the gas, and thus may not be suitable in applications in which the composition of the gas may change, or where the gas may be subject to varying water vapor content. A flow sensor system that addresses and compensates for these additional sources of error is disclosed in U.S. Pat. No. 5,263,369--Cutler, assigned to the assignee of the invention disclosed and claimed herein.
Nevertheless, there is a continuing need, especially in medical ventilator systems, for improved accuracy and stability in gas flow measurements, whereby measurement errors due to variations in ambient gas temperature, gas transport properties, and gas mixture composition are further minimized as compared with the prior art. There is also a need to provide such improved accuracy and stability without adding undue complexity, and thus keeping manufacturing costs low.